The energy of a battery increases in proportion to energy density. In addition, as conversion of a secondary battery into one having higher energy proceeds, the safety of a battery is threatened. In a normal state, electric energy stored in a positive electrode and electric energy in a negative electrode are maintained separately and safely by a separator. However, a short-circuit between the positive electrode and the negative electrode may be caused by various factors to emit the stored electric energy in a short time, thereby causing a heat emission/ignition or thermal runaway phenomenon. Particularly, in the case of a lithium secondary battery, when a high degree of current flows in the battery within a short time due to an internal short-circuit caused by nail penetration, the battery is heated by heat emission and there is the risk of ignition/explosion.
Therefore, as a part of attempts for ensuring the safety against nail penetration, a method for using a device mounted to the outside of a cell and a method for using a material inside of a cell have been studied and used mainly. A positive temperature coefficient (PTC) device using a change in temperature, a protective circuit using a change in voltage and a safety vent using a change in internal pressure of a battery correspond to the former, while addition of a material capable of being changed physically, chemically or electrochemically depending on a change in temperature or voltage in a battery corresponds to the latter.
Since the devices mounted to the outside of a cell use temperature, voltage and internal pressure, they can bring about confident interruption (breakage) but require an additional installation process and installation space. In addition, it is known that such devices cannot perform a protective function sufficiently, when a rapid response time is required, for example, in the case of an internal short-circuit, nail penetration, local damage, or the like.
As a method of using a material in a cell, studies about addition of a chemical safety device including addition of an additive capable of improving safety to an electrolyte or electrode have been conducted. Such a method is advantageous in that it does not require a space and can be applied to any types of batteries. However, it is reported that a material forming a passive film on an electrode is produced or volumetric swelling occurs when the temperature is increased, thereby increasing the resistance of an electrode. In each case, there are problems in that byproducts are generated upon the formation of a passive film and the performance of a battery is degraded, or the volume occupied by the material in a battery is large to cause a decrease in the capacity of a battery.
Therefore, there is still a need for developing a technology for preventing heat emission or ignition caused by nail penetration.